1. Field of the Invention
Embodiments relate to an electrode structure for a solar cell having a transparent electrode.
2. Description of the Related Art
Some thin-film type solar cells include a transparent electrode, a p-n junction and a metal film. The transparent electrode is generally made of a transparent material that allows sunlight to pass through and has excellent electric conductivity characteristics. The p-n junction is formed of a p-type semiconductor material and an n-type semiconductor material. The p-n junction may be in the form of a p-i-n junction that includes an undoped layer of intrinsic semiconductor material sandwiched between a layer of p-type semiconductor material and a layer of n-type semiconductor material. The metal film is in contact with the semiconductor materials to convey current generated by exposing the solar cell to the sunlight.
FIGS. 1A through 1D are schematic views of conventional thin-film type solar cells. As illustrated in FIGS. 1A through 1D, the thin-film type solar cells may be in the form of a basic structure where a transparent electrode, a semiconductor junction, and a back metal contact are stacked on top of each other. The thin-film type solar cell may further include an anti-reflection coating, cover glass or the like. The semiconductor junction generally is a p-n junction, a p-i-n junction or any combinations thereof. The transparent electrode is often formed of a transparent conducting oxide (TCO).
In order to form the back metal contact, a metal film such as molybdenum (Mo), nickel (Ni) or aluminum (Al) is first deposited on a substrate such as a float glass or a metal foil. Then, an amorphous, micro-crystalline or poly-crystalline semiconductor material is deposited by chemical vapor deposition (CVD) (e.g., thermal CVD or plasma-enhanced CVD) or physical vapor deposition (PVD) (e.g., sputtering or evaporation). The back metal contact may form a Schottky contact between a metal and an n-type semiconductor material.
FIG. 1E is a schematic view of a conventional bulk type solar cell. In the bulk type solar cell uses a polycrystalline or single crystalline wafer. The wafer is obtained from an ingot doped with boron (B). Thus, an n-p junction type structure is formed by doping phosphorous (P) into a p-type semiconductor. In order to form a back metal contact, a p-p+ junction layer is made by performing a heavy doping process with respect to the back of the structure. The back metal contact forms a back surface field. Accordingly, the structure comes in ohmic contact with a metal film formed of Al, Mo or the like.
Visible light spectrum takes up about 40% or more of the energy of the sunlight. Therefore, a solar cell may have a band gap of about 3 electron volts (eV) so as to have transparency in the visual region. The solar cell may have a relatively high electric conductivity so that current is generated by the movement of electrons or holes. Thus, a degenerated n-type oxide transparent semiconductor such as indium tin oxide (ITO), zinc oxide doped with aluminum (ZnO:Al) or tin oxide doped with fluorine (SnO2:F) may be used as a transparent electrode of the solar cell. The ZnO has a band gap of about 3.4 eV. The ZnO also has advantageous characteristics of relatively high transmittance, relatively high electric conductivity in the visual region, and low price. Hence, the ZnO may be used as the substitute of an ITO film. In order to improve the electric conductivity, a degenerated n-type oxide transparent semiconductor may be formed by doping Al into the ZnO.
In a thin-film type solar cell or display device, a semiconductor junction layer, a metal layer and a transparent electrode film are formed by deposition, and hence, a Schottky junction can be used. In the structure in which an ITO or ZnO:Al film is formed adjacent to the p-layer of a p-n junction or a p-i-n junction, holes generated by sunlight is annihilated in a transparent electrode layer because the transparent electrode layer is an n-type oxide semiconductor. Thee annihilation of the holes tends to lower the efficiency associated with generating current or carriers (e.g., electrons or holes) by exposing the solar cell or the display device to the sunlight.
Furthermore, a thin film obtained by CVD or PVD may have degraded physical, chemical and electrical properties due to defects, impurities, defects of crystalline structure, imperfect grain boundary, or the like. The degraded properties is detrimental to generation and diffusion of carrier due to the sunlight, and results in poor performance associated with drift and the like.
As a result, the thin-film type solar cells generally tend to exhibit lower short circuit current and lower efficiency compared to the bulk type solar cell and are more susceptible to light induced aging.